In the past, for recording information upon a recording medium such as a holographic memory or the like, first the information which is to be recorded is coded into a two-dimensional image, and then this information, coded in the two-dimensional image, is recorded upon a recording medium (a recording material) by using a laser beam or the like as a signal beam or a reference beam.
The original information is then reproduced by, for example, decoding a two-dimensional image which has been obtained by an imaging device by using a laser light beam or the like as a reference beam.
As a method of representing information two-dimensionally in this manner, there is an information coding method of representing 2 bit information by 4×4 pixels (refer to Japanese Unexamined Patent Application, First Publication No. 2002-366014—hereinafter termed Patent Document #1). Furthermore, there is a distortion compensation method of compensating the size of an image which has been detected back to the original image size (refer to Japanese Unexamined Patent Application, First Publication No. 2003-78746—hereinafter termed Patent Document #2).
As is described in these documents, normally, it is often the case that the information is recorded as a binary image made up from black and white. Furthermore, the reproduction of the information which has been recorded is performed by capturing an image with an imaging device such as a CCD (Charge Coupled Device) camera or the like, and by thus reading out the information which is recorded.
When representing information by a two-dimensional image in this manner, if the two-dimensional image is represented as a binary image made up from black and white, the amount of information which can be represented in a two-dimensional image of a given size is, at the maximum, the number of bits corresponding to the number of pixels in that image.
However, since during reproduction the image is captured with an imaging device such as a CCD camera or the like, the pixels of the CCD camera and the pixels of the recorded image deviate from one another, and hence it may happen that the information is not properly read out. Therefore, in practice it is difficult to represent one bit of information with one pixel of the CCD camera.
For example, if a white, a black, a white, and a black pixel are present in that order along a line in the recorded image, then, if this image is captured by the CCD camera in a state in which it has deviated by exactly 0.5 of a pixel, a gray color, which is a color between white and black, is obtained at a CCD pixel which is positioned between a white pixel and a black pixel. Thus, the line consists of a gray, a gray, and a gray pixel is obtained, so that it has become impossible to extract the original information. In other words, when information which is meaningful is arranged at the pixel pitch of the imaging device, it becomes difficult to read it out due to the influence of positional deviation and the like.
Furthermore, as a method of solving this problem, Patent Document #1 proposes a coding method in which two bits of information are represented by 4×4 pixels. However, since this method represents 2 bits by 16 pixels, the coding rate is ⅛, and thus there is the problem that the amount of information which can be recorded is low.